Inductively coupled plasma atomic emission spectrometry ("ICP-AES") and inductively coupled plasma mass spectrometry ("ICP-MS") are elemental analysis techniques widely utilized throughout the world. Although ICP-MS instrumentation is of relatively recent origin, the use of this technique is gaining rapidly since significantly superior detection limits can be achieved with ICP-MS over those obtainable with ICP-AES. Both techniques rely on the ability to convert liquid samples into a form which is compatible with an inductively coupled plasma ("ICP"), which is the final component of the techniques just prior to the analysis provided by the spectrometer.
The ICP, which is effectively a high temperature source with effective temperatures ranging from 7,000 K. to 10,000 K. at various zones within the plasma, is basically used as a torch. The torch serves as a source for either generating excited atomic particles for emission spectra or ions for mass analysis. Sample introduction into the ICP is considered the problematic step associated with these techniques.
The most common technique for introducing liquid samples is as aerosols produced by pneumatic nebulizers. Although there are several designs of pneumatic nebulizers, these only have efficiencies of approximately 1-2 percent with the particles in the resulting aerosol having an average diameter of 10-20 microns. Prior to introduction of the aerosol into the plasma, the aerosol must be subjected to several processes before atomic emission and ionization is observable.
Since the solvent which was utilized to generate the liquid sample will unnecessarily cool the plasma and thus reduce the efficiency of the plasma to produce excitation and ionization, the aerosol preferably should undergo desolvation, which strips the solvent from the sample aerosol particles leaving behind a dry aerosol. Following desolvation, the dry aerosol is vaporized in the ICP torch to produce free excited atoms and ions. The vaporization process is inherently affected by the size of the aerosol particles; the smaller the aerosol particles, the more efficient the vaporization process.
Relatively recently, a more efficient nebulizing system in the form of an ultrasonic nebulizer ("USN") has been developed. An USN generally satisfies most of the criteria required for improving the aerosol producing stage of the sample introduction system of an ICP-AES or an ICP-MS. Typically, an USN system has an efficiency of greater than 10 percent and produces an aerosol of particles having sizes ranging from one to three microns in diameter. Unfortunately, the higher efficiency of an USN introduces substantially more solvent into the aerosol with resultant degradation of plasma performance due to cooling, thereby increasing the need for a more efficient desolvation system.
In accordance with the present invention, a desolvation system for aqueous or organic solvents comprises a condenser and preferably a low temperature furnace assembly and one or more condensers connected in series. The furnace is normally operated at a temperature which is sufficient to rapidly convert all suspended liquid particles into vapors. The sample vapors exiting the furnace are directed into the condensers, the number of the condensers depending on the solvent or solvent mixture being analyzed. The function of the condenser is to rapidly strip the solvent vapors from the aerosol stream. To accomplish this goal, the condensers must be maintained at temperatures ranging between -100.degree. C. and 0.degree. C., depending on the solvent.
Prior systems have been developed to remove solvent from the sample stream; however, these systems have not been sufficiently effective to consistently produce a relatively solvent-free dry sample to be delivered to the plasma generating region. Such a prior art system is described in the paper by Fassel and Bear entitled "Ultrasonic Nebulization of Liquid Samples for Analytical Inductively Coupled Plasma-atomic Spectroscopy: an Update"; published in Spectrochim Acta, Vol. 41B, No. 10, pages 1089 through 1113, 1986, which is incorporated herein by reference to further illustrate overall systems.
The major problem with prior art systems has been the failure of the condenser to perform as needed within the system. Several criteria must be met when designing a practical and high performance condenser, such as (1) providing a compact physical size for the condenser; (2) rapidly cooling the aerosol to minimize nucleation of the desolvated analyte particles with the solvent particles; (3) providing an aerosol flow which is laminar or which has minimal turbulence in order to minimize the nucleation or coalescing of small desolvated analyte particles into larger particles; and (4) rapidly removing the condensed solvent from the condenser in order to prevent reintrainment of solvent vapors in the carrier gas. The present invention is designed to satisfy the above criteria.